Plate for electrostatic electrophotography



Feb. 8, 1966 s oc E 3,234,020

PLATE FOR ELECTROSTATIC ELECTROPHOTOGRAPHY Original Filed Jan. '7, 1958g 'llllllllllllllllllllln o INVENTOR.

JERRY L STOCKDALE ATTORNEYS United States Patent 3,234,020 PLATE FORELECTROSTATIC ELECTROPHOTOGRAPHY Jerry L. Stockdale, Indianapolis, Ind,assignor, by mesne assignments, to Xerox Corporation, Rochester, N.Y.,

a corporation of New York Continuation of application Ser. No. 707,561,Jan. 7, 1958.

This application June 21, 1961, Ser. No. 118,626 6 Claims. (Cl. 96-11)This invention relates to xerography and in particular to an improvedxerographic plate; and this application is a continuation-impart of myapplication Serial No. 235,383 filed July 5, 1951 (now abandoned) and acontinuation of my application Serial No. 707,561, now abandoned, filedJanuary 7, 1958. v

In the art of xe-rography, which was first presented in Carlson US.Patent No. 2,297,691, an electrophotographic reproduction is made byplacing an electrostatic charge pattern or electrostatic image on aninsulating surface such as a photoconductive insulating surface andthereafter developing the electrostatic image by means of anelectrically-attractable material.

This invention is an improvement in the art of xerography as disclosedin the aforesaid Carlson patent of such significant character that ithas now become feasible to apply the process to continuous-toneelectrophotography wherein the electrostatic image runs a continuousgradation of potential corresponding to a continuous gradation in theoptical density of the subject to be reproduced.

In accordance with the present invention, there is provided an improvedxerographic plate which is capable of reproducing an optical imagefaithfully and in pleasing tone rendition. In a process for which thisimproved member is particularly useful, a xerographic member comprisinga photoconductive insulating layer on a conductive backing is providedwith a uniform electrostatic charge on its photoconductive layer and isthen exposed to a light image. On exposure to the light image, thephotoconductive layer becomes conductive in selective areascorresponding to the bright area of the image, and

' the electrostatic charge is dissipated preferentially in these areas.To meet the high requirements of continuous-tone electrophotography, itis essential that the electrophotographically-sensitive member becapable of accepting a uniform charge both in terms of uniform surfacepotential and in terms of total charge per unit area, and it is furtheressential that the photoconductivity of the sensitive member be uniformthroughout the area and be generally proportional to the lightintensity. Without these strictly exact characteristics, adequatecontinuous gradation of tone is unlikely and the sensitive member wouldbe suitable only for line copy reproduction or for relatively crudecontinuous-tone work.

It is, therefore, an object of this invention to provide an improvedxerographically-sensitive member suitable for fine grade continuous-tonereproduction, and to provide a process for the preparation of saidimproved member.

It is a further object of this invention to provide an improvedelectrophotographic member comprising a photoconductive layer on aconductive backing, this member being of high uniformity suitable forfine continuous-tone reproduction, and to provide a process for thepreparation of such improved member.

In the production of a xerographic member suitable forelectrophotography, any backing may be provided for this photoconductiveinsulating material which possesses greater conductivity than the darkconductivity of the photoconductive insulating layer, as stated in theaforesaid Carlson Patent No. 2,297,691, wherein reference is made to theuse of metal plate backings such as zinc, aluminum,

3,234,020 Patented Feb. 8, 1966 and brass. This patent also mentions theemployment of other ba-ckings such as paper, glass, plastic or othersheets which may be impregnated or coated with a conductive materialsuch as a conductive metal, metallic-conductive compounds, carbon andthe like, both with and without some binder material. In thexerogra-phic member, the backing serves the primary purpose of providinga conductive material in continuous, electrically coupled relation withthe layer of photoconductive insulating material whereby wherever thelayer of photoconductive insulating material is rendered conductiveresponsive to exposure of the photoconductive insulating material tolight, and to the extent of such exposure, an electrostatic chargepreviously imposed on the layer of photoconductive insulating materialmay become dissipated with resultant production of an electrostaticimage. Accordingly, many other backing plates may be employed inaddition to those that have been exemplified. However, especially in thecase of continuous tone reproduction, it is important that theelectrical coupling between the backing and the layer of photoconductiveinsulating material be highly uniform throughout the area of theelectrostatic latent image. Moreover, it is important that an originalelectrostatic charge be retained to the maximum extent possible and withmaximum uniformity of retained charge during the interval between thecharging of a xerographic member and the exposure of the member to lightfor the production of the desired photoelectric image.

The aforesaid Carlson Patent No. 2,297,691 gives a number of examples ofphotoconductive insulating materials which may be utilized in aXerographic member. Among them are sulfur, anthracene, anthraquinone,melted mixtures of sulfur and selenium with the sulfur predominating,and certain mixtures of sulfur with anthracene or linseed oil. Morerecently it has been proposed to employ as and for the layer ofphotoconductive insulating material a layer of substantially pureselenium that has been produced under such conditions that the seleniumis not in the semi-conductor metallic state mentioned in the saidCarlson patent but is in an amorphous, vitreous, photoconductiveinsulator state, such as that which results from the procedure disclosedin application Serial No. 221,042, filed April 14, 1951, entitled,Xerographic Member, and Method and Apparatus for the Production Thereof(now Patent No. 2,753,278, issued July 3, 1956). Such vitreous seleniumphotoconductive insulating material is considerably moreelectroconductively responsive than the materials mentioned in theCarlson patent, thus making it possible to utilize shorter periods oflight exposure or less intense light exposure whereby greater all-aroundutility and suitability for continuoustone are afforded. Accordingly, itis preferable to employ selenium for the photoconductive insulatingmaterial although in practising this invention essentially the sameutilization of photoconductive insulating properties in involved in theproduction of a photoelectric latent image, whether selenium is used orsome other photoconductive insulating material. The selenium that isused is sub stantially pure and does not contain the small amount ofhalide that is essentially present to impart conductive characteristicswhen selenium is used for other purposes wherein substantialconductivity is a required characteristic.

Features of this invention involve the cleaning of a surface area of anelectroconductive backing member which is free of substantially allforeign matter, thereby leaving the surface area substantially uniformthroughout so as to be thatof the substance of which the backing memberis composed, and while the surface is freedof all foreign matter, asaforesaid, causing a film of insulating material to be distributed overthe surface area in directly adherent relation therewith with essentialuniformity of both composition and thickness. An essentially uniformlayer of photoconductive insulating material thereafter is applied tothe film of'insulating material. The film of insulating material is sodistributed in relation to the elements on the opposite sides thereofthat the xerographic plate as a Whole has a substantially decreased darkdecay rate as compared with a corresponding plate wherein the surface ofthe backing member has been similarly cleaned without, however, applyingthe film of insulating material. At the same time, the plate permitseffective discharge of an electrostatic charge upon increase in theconduc tivity of the layer of photoconductive insulating materialresponsive to exposure to light or other radiation that is effective toimpart increased conductivity to the photoconductive insulating materialthat is used. In preferred embodiments of this invention, theintervening barrier film does not substantially decrease the rate oflight decay of the plate or otherwise affect the responsiveness of theplate to light exposure and may actually increase the rate of lightdecay.

By utilization of this invention, very significant improvements inxerography have been obtained, especially in the attainment of improvedcontinuous-tone reproductions. The sensitivity of a xerographic plate tolight and to the development of electrostatic image contrasts isimpaired by loss of an initially imparted electrostatic charge and,inasmuch as there is always a substantial interval of time between theimparting of the charge and the light exposure whereby the latentelectrostatic image is produced and between the light exposure anddevelopment of the plate, the decrease in dark decay rate that isobtained according to this invention is of major practical significanceas regards its effect on the quality of the reproductions produced.Moreover, both any dark decay as may occur and the light decay responseto illumination are caused to be more uniform throughout the surfacearea that has been treated according to this invention with the resultthat the electrostatic image that is produced more nearly parallels thedifferences in illumination and avoids inaccuracies due to the effect onthe electrostatic image of factors such as non-uniform charge retentionand nonuniform response to illumination.

In the drawing the figure is a diagrammatic representation partially insection of an improved xerographic member according to this invention.In the figure there is shown a new xerographic member comprising abacking plate 11 having on its surface an extremely'thin interfaciallayer 12 with a top layer of photoconductive vitreous-appearing selenium13.

The general nature and scope of the invention having been set forth, thefollowing specific examples are presented in illustration but not inlimitation of the invention, and it is to be understood that theinvention is limited only by the appended claims.

Example 1 A brass plate having a surface area about 4 x 5 inches andcharacterized by a mirror-smooth surface free from visible flaws andimperfections was selected as a base plate for a xerographic member.This brass backing plate was carefully prepared by the followingcleaning steps: The surface was coated with a commercial wax base polishbelieved to be an aqueous emulsion of a paraffin hydrocarbon wax and thesurface was polished with a soft cloth; the polished surface was Washedwith water containing a small amount of a Duponol (a trade name for ahigher aliphatic alcohol sulfate); the surface was thoroughly rinsedwith water and then with methyl alcohol; and the surface was immediatelydegreased by suspending it in isopropyl alcohol vapor.

The thoroughly cleaned brass base plate was then placed in a vacuumevaporation chamber with its rear surface mounted against a temperaturecontrol platen through which was circulated water at a controlledtemperature and substantially pure selenium was evaporated onto thecleaned surface of the plate according to the procedure claimed incopending application Serial No. 221,042, filed April 14, 1951, andentitled, Xerographic Member, and Method and Apparatus for theProduction Thereof. According to this procedure, the brass plate wasmaintained at a temperature of about 65 C. and a SO-micron layer ofvitreous selenium was evaporated thereon during a period of about tenminutes. Uniformity of thickness of the selenium layer was assured byevaporating the selenium simultaneously from four evaporation boatspositioned substantially directly opposite the corners of the brassplate, whereby the film was of equal thickness over the entire areawithin a tolerance of 2%. The resulting plate was characterized by beingexcellent for the reproduction of continuous-tone images by thezerographic process.

Example 2 The procedure of Example 1 was repeated employing thefollowing cleaning steps on the brass plate: The plate was firstthoroughly washed with water containing 1% Duponol and then evaporateddry. An 8% solution of parafiin wax in benzene was applied to thesurface with a cotton swab and air dried. This was then rubbed brisklyuntil polished and washed again with 1% Duponol. The surface was thenrubbed with water and flushed with methanol, after which it wasimmediately degreased with isopropyl alcohol vapor. The plate was thenplaced in a vacuum chamber and coated with selenium, according to theprocedure of Example 1, to yield a satisfactory continuous-tonexerographic plate.

The rigorous cleaning steps described in the examples are believed toperform two functions. In the first plate, these cleaning steps aredesigned to remove completely all films of dirt, grease, oxides, stainsand the like from the surface of the plate, and, in the second plate, itis believed that these steps leave on the surface of the plate a verythin uniform layer of the cleaning agent, in the specific examplesdescribed, this being a wax cleaning agent, employed in the cleaningsteps. Thus, it is believed that there remains a thin coating,presumably a very few molecules thick, of the cleaner component. Thepresence of the residual thin interfacial film or barrier layer has beenestablished experimentally by its efiectiveness in preventingamalgamation of a brass test plate of the character aforesaid withmercury placed therein. Thus three like brass test plates were cleanedby successively treating them with hydrochloric acid, isopropyl alcoholand hot isopropyl alcohol vapor. Two of the three plates had Glass Waxapplied thereto which was subsequently re- .moved with a dry cottonswab. One of the blanks that had had the Glass Wax applied thereto wasdegreased by treating it for five minutes in hot isopropyl alcohol vaporfor five minutes. A drop of mercury was placed on each of the threeplates. The mercury did not wet the surface of either of the plates thathad had Glass Wax applied thereto but did amalgamate with the surface ofthe plate that had not had the Glass Wax applied thereto, therebydemonstrating that a barrier film is left by the Glass Wax and that thefilm is not removed by a degreasing treatment of the kind employed inthe preparation of the plate of Example 1.

Example 3 Three brass test plates were thoroughly cleaned by treatmgeach with successive application of 1.2 N hydrochloric acid, isopropylalcohol, and hot isopropyl alcohol vanor. To the surface of the cleanplates 21 film was applied donsisting of polystyrene, by dipping theplate in a toluene satisfactory continuous-tone xerographic plates.

. Example 4 Two brass test plates were thoroughly cleaned by treatingeach with successive applications of 1.2 N hydrochloric acid, isopropylalcohol, and hot isopropyl alcohol vapor. To the surface of one of thecleaned plates a film was applied consisting of Lucite 46, which is atrade name of E. I. du Pont de Nemours & Co. for a polymeric butylmethacrylate, by dipping the plate in a toluene solution of the Lucite,theresultingfilrn being about 3 microns in thickness. After storage ofthe film coated plate for 72 hours, both plates were coated with acoating about 45 microns in thickness of amorphous selenium by vacuumevaporation at about 73 C. and about 1 micron of mercury. The platehaving the Lucite interfacial film exhibited much improved resistance todark decay, without impairment of light sensitivity, as evidenced by thefollow data:

Initial Dark Light Residual Intsrfacial layer potent. decay, sensitivitypotent,

(positive), percent (400 mp) v.

Lucite (3 555 0.72 17. 5 26 None (control) 500 4 17. 5 25 In theforegoing table and elsewhere herein dark decay expressed in percent iscomputed as follows:

(V V X 100 Percent dark decay= where T is the time in seconds requiredfor the plate potential to decay under the light exposure to one-half ofits initial value, V is the initial potential and I is the lightintensity.

Example Two brass plates were prepared as described in Example 4 andwere placed in a common evaporator having separate shutters forscreening each of the plates. While maintaining about 1 micron mercurypressure, a film of sulfur substantially 1.5 micron in thickness wasapplied to one of the plates while the other plate was screened by itsshutter. Thereafter, without breaking the vacuum, a coating about 45microns in thickness of amorphous selenium was applied at about 73 C. toboth of the plates. The plate having the sulfur interfacial filmexhibited much improved resistance to dark decay while possessingsatisfactory light-sensitivity, as evidenced by the following data:

resistance to dark decay without adversely affecting lightsensitivity;

Initial Light sensitivity Interfacial layer potent. Dark decay (400 my)(positive), v.

S (3,) 460 440 v. 30 min. Fell to 30 v. in

later. 1 min. None (control) 450 350 v. 30 min.

later.

Example 7 Initial Light sensitivity Iuterfacial layer potent. Dark decay(400 m (positive), v.

GazSca (1p) 440 400 v. 30 min. Fell to 14 v. in 1 later. min. and 0.5 v.

in 25 min. Ga Sea (Z-3 560 520 v. 30 min. Fell to 22 v. in 1 later. min.and 3 v.

in 10 min.

Example 8 Two aluminum plates were thoroughly cleaned, first usingisopropyl alcohol and then hot isopropyl alcohol vapor. One of thecleaned aluminum plates was coated with a Lucite film about 3 microns inthickness as described above in Example 4. The interfacial film ofLucite resulted in a very good improvement in resistance to dark decaywithout impairing light-sensitivity, as evidenced by the following data:

Initial Dark Light sen- Interfacial layer potent. decay, sitivityResidual (positive), percent (400 m potent. v.

Lucite 46 (3,u) 459 3. 9 19 25 None (c0ntrol). 430 40 18. 5 16 Example 9Two brass plates which had been cleaned as described above in Example 4were placed in a vacuum evaporator and while maintaining a pressure ofabout 1 micron of mercury each of the plates had applied thereto byvacuum deposition a film of aluminum of about .3 micron in thickness.Without breaking the vacuum, one of the aluminum coated plates haddeposited thereon a film Interfaciamyfl Dark decay zf gg i g of sulfurabout 1 micron in thickness. Thereafter each (positive),v. of the plateshad deposited thereon a coating about 45 microns in thickness ofamorphous selenium. In this s (1.5 580 520 v.,30 min. Fell to 30 v.in1instance also there was a pronounced improvement in N0ne 580 430mmresistance to dark decay while light-sensitivity remained later. high,as evidenced by the following data:

Example 6 Interfaeial layer g ii iiii. sii t igi t y Residual Two brassplates were prepared as described in Expercent (400 potent"v ample 5except that the sulfur interfacial film was applied so as to besubstantially 3 microns in thickness. As evi- S (1) 567 26 132 denced bythe following data, the increased thickness of None n 00 11 16 28 a 7Example A coating of chromium about 25 microns in thickness was appliedelectrolytically to two brass plates, which were thereafter subjected tothe cleaning procedure hereinabove described in Example 4. Also asdescribed in Example 4, a coating of Lucite about 3 microns in thicknesswas applied to one of the plates and thereafter each of the plates hadapplied thereto by vacuum deposition a coating of amorphous selenium. Inthis instance a very great improvement in resistance to dark decayresulted from the Lucite interfacial film, there being as well someincrease in light-sensitivity. The data in connection with this exampleappears below:

Initial potent. (positive), v.

Dark decay. percent Light Residual scnsit. potent,

Fatigue Lucite 46 (3 1) None (control).

Example 11 ,Were also tested for fatigue and much improved resistance tofatigue was noted in the case of the plate having the interfacial Lucitefilm. The data in connection with this example is as follows:

Initial potent. (positive) v.

Light sensitivity Interiacial layer Fatigue, percent Lucite 46 (3 32 8.2 None (control) 8. 3

Fatigue as given in the above table and elsewhere herein is obtained bydividing the difference in voltage before and after exposures toradiation (in each case taken three minutes after charging) by thevoltage on the plate before exposure (three minutes after charging), thevalue of the ratio being expressed as a percentage.

Example 12 The procedure of Example 10 was repeated except for thevacuum deposition of a film of indium on the brass plates instead ofgold, the results of this test being as follows:

Light sensitivity (450 mp) Initial potent. (positive), v.

Dark decay, percent Intertacial layer Fatigue,

percent Lucite 46 None (control) Example 13 Example 8 was repeatedexcept that the plates were subjected to a negative potential ratherthan a positive potential and the following data was obtained:

Dark decay, percent Initial potent. (neg), v.

Light sensitivity Interiacial layer Lucite 4G (3p) None (control) ll 30l 'surface of the base plate.

8 Example 14 Example 9 was repeated except that the plates weresubjected to a negative potential rather than a positive potential andthe following data was obtained:

Example 15 The plate of Example 4 having the Lucite interfacial layerand the plate of Example 6 having the sulfur interfacial layer were eachsubjected to a uniform electrostatic charge comparable to that imposedin ordinary commercial xerography. Both plates were then stored in thedark for three days. At the end of this period each of the plates wasexposed to a light image and was developed using normal xerographicprocedures. The quality of the developed image was fully equivalent tothat obtained using conventional plates when such plates are exposed anddeveloped immediately after charging. The capacity thus exhibited forresisting dark decay for three days represents a very greatimprovementas compared with conventional plates previously employed inxerography.

The'function of the interfacial layer is not known with certainty but itis believed that in addition to protecting the surface from chemicalaction immediately prior to the coating operation it also acts as aninsulator or at least as a barrier between the layer of photoconductiveinsulating material and the electroconductive substance of which thesurface of the backing is composed. It is known, for example, that invitreous selenium the mechanism of charged migration is attributable tothe migration of positive holes, that is, such electrical conductivityas it possesses occurs in the form of the travel of apparent electronspaces or holes within the layer rather than by the travel of freeelectrons through the layer. It also is known that the preferredxerographic steps employed in connection with selenium as thephotoconductive insulating material include imparting a positivepolarity surface charge to the photoconductive layer, which positivecharge is dissipated upon exposure to light and is believed to beconducted through the layer'to the conductive base plate or backing.Basing theory upon these facts, it is believed and understood that theinterfacial film or layer of insulating material that, acoording to thisinvention, is provided between the electroconductive base plate and thelayer of selenium or other photoconductive insulating material serves toprevent premature transmission of the electrons to the photoconductivelayer prior to its exposure to light. Whether this theory is correct ornot, it has been found that in the absence of the application of theinterfacial film of insulating material the resulting xerographic plateis characterized by excessive loss of charge in the absence mottled and,therefore, unsuited for continuous-tone electrophotography.

The interfacial film which is employed according to this invention iscomposed of insulating material in contrast with the electroconductivesubstance presented at the The insulating material of which theinterfacial film is composed also has less conductivity than the darkconductivity of the overlying layer of photoconductive insulatingmaterial in the sense that when the plate is charged and is not exposedto activating radiation an insulating barrier is provided which lessensthe rate at which the potential imposed on the layer of photoconductiveinsulating material becomes dissipated into the base plate. Theinsulating material of which the interfacial film is composed ordinarilyis not a photoconductive insulating material whose conductivity isincreased upon exposure to radiation. However, such increase inconductivity upon exposure of the plate to activating radiation is notinconsistent with the practice of this invention inasmuch as it isdesirable rather than otherwise that an imposed charge be dissipatedrapidly in those areas that are exposed to activating radiation. Thus,sulfur possesses photoconductive insulating characteristics to a certainextent but may very advantageously be used as the material of theinterfacial layer in combination with an overlying layer ofphotoconductive insulating material such as vitreous selenium. However,the invention contemplates the employment of an interfacial film orbarrier layer composed of some material which is different from thecoating of photoconductive insulating material that is primarily used toreceive the electrostatic charge and on which the electrostatic image isproduced.

As used in the xerographtic process an electrostatic field is placedacross the photoconductive insulating layer. The interfacial film orbarrier layer defined in the instant invention is an insulating film inthe sense that it prevents the injection of carriers from the base plateinto the photoconduotive insulating layer under the influence of theapplied field. Thus, such a layer may act to prevent the injection ofboth positive and negative charge carriers. However, as xerographicplates are normally used with only one polarity of changing, the barrierlayer may prevent the injection of only one polarity of charge carrier.Thus, a vitreous selenium plate is normally used with a positive fieldapplied to the surface thereof. Accordingly, the barrier layer shouldact to prevent the injection of negative charges from the backing memberinto the selenium. If the selenium contains a quantity of arsenictrisulfide, the resulting xerographic plate is normally used withnegative sensitization. For such a photoconductive insulating layer thebarrier layer should prevent the injection of positive charges or holesfrom the hacking into the photoconductive layer. The thickness of thebarrier layer is determined by two factors: First, the difiiculty ofobtaining a uniform interfacial film, and, secondly, the fact that thefilm must not be so thin that tunneling occurs to such an extent as toobviate the effectiveness of the barrier layer. For most materials thismeans that the interfacial film should have a thickness of at leastabout 0.1 micron. For certain materials such as those applied in theGlass Wax treatment shown in Example 1, there is "apparently an abilityto strongly and uniformly wet the surface of the brass backing, therebycreating a higly uniform layer even under conditions wherein the layeris extremely thin. The layer in those cases is no more than about 100 orso angstroms thick. the barrier layer is generally determined by themaximum residual voltage that can be tolerated under conditions of useof the plate. The formula for determining this follows from the factthat the capacity of the photoconductor is to the capacity ofthe barrierlayer as the initial potential applied to the plate is to the residualpotential. Assuming an initial potential of 500 volts applied to aselenium photoconductor, then 500 divided by the residual voltage equalsthe thickness in microns of the selenium times the dielectric constantof the interlayer divided by the thickness of the insulator times thedielectric constant of the selenium. For a SO-micron selenium plate, andan interlayer material with a dielectric constant of about 3, then theresidual voltage is equal to 20 times the thickness in microns of theinterlayer. It can be seen from this proportion that the larger thedielectric constant of the interlayer, the greater the permissiblethickness of the interlayer. In general, it is desirable that theinterlayer not be thicker than about microns.

Apart from the presence of the thin interfacial layer The limitingfactoron the maximum thickness of of waxy or other insulating material,it is essential that the surface area of the base plate to be used forelectrophotography be scrupulously cleaned prior to the coatingoperation so that complete absence of any other interfacial material isassured. Dirt, moisture, spots, finger prints and other surface marks onthe surface of the base plate, including even marks so slight as to besubstantially invisible to the naked eye, have been found to bedetectable through the coating in the form of substantial defects inxerographic prints produced from the plate. Accordingly, the surfacepresented by the electroconductive substance of the base plate should becleaned so as to be essentially free of all foreign matter other thanthe electroco-nduotive substance of which the surface area is composed.

In order to maintain the uniformity of results essential to fine qualitycontinuous-tone xerography, it also is necessary that the selenium layerbe of substantially uniform thickness. In the absence of suchuniformity, the xerographic results are impaired in two ways. In thefirst place, the charge acceptance of the selenium layer is uneven sothat a charge of non-uniform potential 'will be placed on the layer andthis change, furthermore, will be dissipated at a non-uniform rate. Inaddition, because of the varying distance between the surface of thebase plate and the surface of the layer of photoconduotive insulatingmaterial, a varying charge density, or coulombs per unit area, will bepresent even if the potential is made uniform, with the result thatvarying numbers of charged electroscopic particles will be required toneutralize the charge by deposit on the surface. It is, therefore,apparent that uniformity of structure is critically essential to thefine quality demanded for continuous-tone reproduction.

In prior research on xerograph-ic processes and materials it had beenfound that gross permanent defects frequently appeared in vitreousselenium xerographic layers, these defects being apparent after thelayer had been charged to a potential approximating its maximum chargeacceptance or insulation breakdown potential. These defects, appearingas dots about the size of the head of a .pin, were characterized byinability to accept and retain an electrostatic charge, and were of sucha nature as to render the member unsuitable even for line reproductionafter a relatively large number of the spots had accumulated on thesurface. Later, when these gross defects had been substantiallycompletely eliminated by improvements and methods other than those ofthe present invention, it was found that other fine defects weredetectable, these fine defects being substantially unobjectionable inline reproduction but making the member inferior for continuous tonereproduction. In the presently preferred xerographic procedures, axerographic member or plate is charged to a. positive polarity potentialof about 100 volts and exposed to a continuous-tone light image, andunder these conditions it has been found that a typical priorxerographic plate such as, for example, an aluminum plate of clean,degreased mirror finish aluminum (which inherently has a thin aluminumoxide layer thereon) with a 20-micron vitreous selenium layer doesacquire such fine defects whereas the plate according to this inventiondoes not acquire such defects.

In preferred practice of this invention, the coating of photoconductiveinsulating material preferably is composed of vitreous selenium. Thephotoconductive insulating layer shouldbe between about 20 and about 200microns thick, the thickness selected generally depending on theconditions of use of the plate. For line copy work the selenium usuallyis about 20 microns thick and for continuous-tone Work preferably isabout 50 microns thick, although the selenium layer may be somewhatthinner, e.g. about 40 microns, or may be up to about microns inthickness. The selenium layer for the drums of automatic machinesgenerally is about 50 microns thick. For X-ray work the selenium layerusually is about 80 to 160 microns thick. While the nature of theselenium layer has been described as vitreous, the exact molecularstructure is not known, the term being used as descriptive of itsphysical appearance and it is not desired to limit the use of seleniumto any crystalline or allotropic form. It is believed that the seleniumis present substantially in an amorphous form containing minorproportions of a crystalline form of selenium, although it is notdesired to restrict this invention to the presence of such a mixture offorms. It is, therefore, to be understood that the various crystallineor amorphous structures included in the vitreous-appearing form ofselenium are likewise to be included in the meaning of the term vitreousas used herein and in the claims. It likewise is to be understood thatthe term selenium includes not only pure selenium but also selenium thatmay be modified by a controlled amount of an additive that is consistentwith retention of useful photo-insulating effectiveness. While the useof vitreous selenium is preferable, nevertheless xerographic platesusing other types of photoconductive insulating material are improved bythe employment of the interfacial film or barrier layer according tothis invention. Moreover, while excellent results have been obtainedwherein the base plate presents a surface composed of brass,nevertheless other electroconductive substances may be used so long asthe surface thereof is free of visible imperfections and is essentiallyfree of all foreign matter other than the electroconductive substance ofwhich the surface area of the base plate is composed.

The presence of the barrier layer between the photoconductive insulatinglayer and the conductive backing not only reduces dark decay but is alsobelieved to substantially reduce the buildup oftrapped charges uponrepeated use. Upon exposure to activating radiation, hole-electron pairsare created at the point of absorption of the photons. In the case ofselenium this is on or close to the surface. The electrostatic fieldcreated by the sensitizing charges on the surface of the selenium causeone polarity of charge carriers to migrate through the selenium to theconductive backing. As they approach the backing charges of oppositepolarity are injected from the backing to migrate toward the surface.Most photoconductive insulators, such as selenium, cadmium sulfide,cadmium selenide, etc., have a long range for only one range of chargecarrier. Thus, where both polarities of charge carriers are injectedinto the photoconductor, the short range carriers are trapped buildingup a strong residual potential as the plate is reused. The

presence of a barrier interlayer according to the instant invention, bypreventing injection of carriers from the backing, assures that onlythose carriers generated by the incident radiation will be present thusreducing the possibilities for trapping.

What is claimed is:

1. A process for preparing an improved xerographic plate suitable forcontinuous-tone electrophotography, said processcomprising thoroughlycleaning the surface of a smooth brass plate, applying a wax to thecleaned surface and polishing the wax, rinsing thewaxed surface withwater and then with solvent vapor and vacuum evaporating a layer ofvitreous selenium on said surface while maintaining the brass plate at atemperature be- 12 tween about 60 and 75 C., until a uniform vitreousselenium layer, between about 40 and about microns thick, is coated ontothe surface.

2. The process of claim 1 wherein the vitreous selenium layer isproduced at a thickness uniform Within 2% of the average thickness.

3. A process for preparing an improved xerographic plate suitable forcontinuous-tone electrophotography, said process comprising thoroughlycleaning the surface of a smooth brass plate, applying a wax to thecleaned surface and polishing the wax, rinsing the waxed surface withwater and then with solvent vapor and vacuum evaporating a layer ofvitreous selenium on said surface while maintaining the brass plate atemperature between about 60 and C., until a uniform vitreous seleniumlayer, about 50 microns thick, is coated onto the surface.

4. An improved xerographic plate suitable for the production ofcontinuous-tone electrophotographic images, said plate comprising abrass backing member, an interfacial thin coating of a wax produced byapplying a wax coating to the brass surface and thoroughly polishing toremove excess wax, and a layer thereover of vitreous selenium uniformover' its entire area Within a tolerance of about 5% and coated thereonby vacuum evaporation while the brass member was maintained at atemperature between about 60 and 75 C.

5. An improved Xerographic plate suitable for the production ofcontinuous-tone electrophotographic images, said plate comprising abrass backing member, an interfacial thin coating of a wax produced byapplying a wax coating to the brass surface and thoroughly polishing toremove excess wax, and a 40 to 75 micron layer thereover of vitreousselenium uniform over its entire area within a tolerance of about 5% andcoated thereon by vacuum evaporation while the brass member wasmaintained at a temperature between about 60 and 75 C.

6. An improved Xerographic plate suitable for the production ofcontinuous-tone electrophotograp'hic images, said plate comprising abrass backing member, an interfacial thin coating of a wax produced byapplying a wax coating to the brass surface and thoroughly polishing toremove excess wax, and a 50 micron layer thereover of vitreous seleniumuniform over its entire area within a tolerance of about 5% and coatedthereon by vacuum evaporation while the brass member was maintained at atemperature between about 60 and 75 C.

References Cited by the Examiner UNITED STATES PATENTS 2,657,152 10/1953Mengali et al. 2,809,294 10/1957 Vyverberg 96-1 2,901,348 8/1959Dessauer et al. 96-1 2,901,349 8/ 1959 Schatfert. 2,970,906 2/ 1961Bixby. 3,041,166 6/1962 Bardeen.

FOREIGN PATENTS 755,683 8/ 1956 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

A. LOUIS MONACELL, Examiner.

4. AN IMPROVED XEROGRAPHIC PLATE SUITALBE FOR THE PRODUCTION OFCONTINUOUS-TONE ELECTROPHOTOGRAPHIC IMAGES, SAID PLATE COMPRISING ABRASS BACKING MEMBER, AN INTERFACIAL THIN COATING OF WAX PRODUCED BYAPPLYING A WAX COATING TO THE BRASS SURFACE AND THOROUGHLY POLISHING TOREMOVE EXCESS WAX, AND A LAYER THEREOVER OF VITREOUS SELENIUM UNIFORMOVER ITS ENTIRE AREA WITHIN A TOLERANCE OF ABOUT 5% AND COATED THEREONBY VACUUM EXAPORATION WHILE THE BRASS MEMBER WAS MAINTAINED AT ATEMPERATURE BETWEEN ABOUT 60* AND 75*C.